Renewable energy is currently a small portion of society's energy generation compared to fossil fuels energy sources. Although wave energy is a large source of renewable energy, it is an underutilized source because current methods and technologies are not economically competitive. This is due to the high cost of the current technologies or the extensive infrastructure required. In addition, the technology should not pose a threat to marine life or cause other environmental damage. Ideally, the harvested energy may also be stored for use during peak energy demand.
Most wave energy extraction devices known as point absorbers utilize wave motion to move a float relative to another float or body. Two common types of relative motion are employed. The first utilizes float motion relative to rigidly anchored bodies such as sea platforms while the other uses the relative motion of two floating bodies. The large infrastructure cost makes the former type expensive while present known methods of relative motion between two floating bodies have high unit cost. This is because most of these systems need to be designed to withstand the severe marine environment.
Many inventions harvest wave energy by placing floats in the sea and directly attach the linear (heaving) motion of the float to push or pull a piston in a hydraulic cylinder. U.S. Pat. Nos. 582,282 and 888,721 illustrate such a mechanism. Floats are connected to a rod or a lever arm that is connected to double acting cylinder on the other end. In many designs, one end is rigidly anchored to the sea bed while the other is connected to the floats. The motion of the float drives the double acting cylinder to generate energy. Many of these prior articles require a platform or infrastructure anchored to the sea floor. For large scale energy production, these structures have to be designed to withstand high loads at sea which is very costly.
Devices that directly connect the linear motion of a float to the linear motion of a piston pump will only be cost effective for pumping an incompressible fluid such as water or hydraulic fluid. Incompressible fluids are incapable of storing energy unless they are pumped to a high elevation and stored in a dam. Compressible fluids such as air can store energy in the form of pressure. However, a piston pump will not generate sufficient pressure with compressible fluids unless the piston travels a significant portion of its stroke. As ocean waves are unpredictable and irregular, the transmission of wave motion directly to a simple piston will only be efficient in a small range of wave heights. Shallow waves will not generate sufficient compression, and large waves beyond the stroke length of the cylinder are not harvested.
A method described by Leavitt in U.S. Pat. No. 322,129 converts the linear motion of the float to a rotational motion and converted back into a linear motion of a fixed stroke length using a rack and pinion gearing. Such a method would be preferable for compressing air with a piston pump. However, the Leavitt invention requires a structure that is rigidly anchored to the sea floor. In fact, numerous inventions including U.S. Pat. Nos. 738,996; 971,343; 1,045,654; 4,009,395; and 6,269,636 require the use of a platform with permanent foundation for the mechanism to work. These structures, as described earlier, would be costly to build and maintain.
Many designs utilize the relative motion between a float relative to another float or body to harvest wave energy. The relative motion between floats eliminates the need for rigid structures anchored to the sea bed. U.S. Pat. No. 581,067 illustrates such an embodiment of harvesting wave energy. Patented in 1897 by Fletcher, the invention pumps sea water to the shore, where it is used to drive a Pelton wheel to generate energy. A large plate is attached to a submerged float to increase its inertia relative to the surface float. The heaving motion of the float drives the piston in a double acting cylinder combined with one way check valves to pump sea water. As discussed previously, the Fletcher invention only works well for pumping liquid, and pumping sea water have undesirable effects for both marine life and the pump.
Other prior art references use the relative motion of floats to compress a piston pump with hydraulic fluid. The hydraulic fluid is immediately used to drive a small turbine that is typically placed within the float or the point absorber unit at sea. Other prior art references convert the undulating motion of floats to a unidirectional rotary motion using mechanisms such as ratchets or clutches. Many of these inventions that generate electrical energy at sea are inefficient and have high costs. Very few inventions are in practice or generate energy on a large scale, and even fewer are capable of storing the generated energy.